JPH0422497B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal device using ferroelectric liquid crystal that can be applied to a liquid crystal display device or a liquid crystal shutter array.

[Description of prior art]

Conventionally, liquid crystal display elements are well known in which a scanning electrode group and a signal electrode group are configured in a matrix, and a liquid crystal compound is filled between the electrodes to form a large number of pixels to display images or information. . The driving method for this display element is time-division driving, in which an address signal is selectively and periodically applied to a group of scanning electrodes, and a predetermined information signal is selectively applied in parallel to a group of signal electrodes in synchronization with the address signal. It has been adopted.

Most of these were put to practical use, such as "Applied Physics Letters".
(“Applied Rhysics Letter”) 1971, No. 18(4)
“Voltage Tapendant Optical” co-authored by M. Schadt and W. Helfrich, published on pages 127-128.
Activity of a Twisted Nematic Liquid Crystal” (“Voltage”)
Dependent Optical Activity of a Twisted
Nematic Liquid Crystal”)
(twistednematic) type liquid crystal.

In recent years, the use of bistable liquid crystal elements as an improved version of conventional liquid crystal elements has been proposed by both Clark and Lagerwall in Japanese Patent Application Laid-open No. 107216/1983 and US Patent No.
It has been proposed in the specification of No. 4367924, etc. Ferroelectric liquid crystals having chiral smectic C phase (SmC〓) or H phase (SmH〓) are generally used as bistable liquid crystals, and in these states,
It has the property of taking either the first optically stable state or the second optically stable state in response to an applied electric field and maintaining that state when no electric field is applied, that is, having stability. In addition, it is expected to be widely used in fields such as high-speed and memory-type display devices that respond quickly to changes in electric field.

[Problem that the invention seeks to solve]

However, problems arise when the number of display pixels is extremely large and high-speed driving is required.
That is, the threshold voltage for providing the first stable state in a ferroelectric liquid crystal cell having bistability for a predetermined voltage application time is -Vth ₁ , and the threshold voltage for providing the second stable state is -Vth 1. +Vth ₂ ,
Even if these threshold voltages are not exceeded, if the voltage continues to be applied for a long time, the display state written to the pixel (e.g., white state) will be reversed to another display state (e.g., black state) Sometimes. 1st
The figure represents the threshold characteristics of a bistable ferroelectric liquid crystal cell.

Figure 1 shows DOBAMBC as a ferroelectric liquid crystal cell.
Threshold voltage (Vth) required for switching when using (12 in the figure) and HOBACPC (11 in the figure)
This is a plot of the application time dependence of .

As is clear from Fig. 1, the threshold value Vth is dependent on the application time, and the shorter the application time, the more
It is understood that the slope is steep. From this, when applied to an element that has an extremely large number of scanning lines and is driven at high speed, for example, even if a certain pixel is switched to the bright state during scanning, the information signal will always be below Vth from the next scanning onward. It can be seen that if the voltage continues to be applied, there is a risk that the pixel will turn into a dark state during the completion of scanning one screen.

[Means for Solving the Problems] and [Operations] An object of the present invention is to provide a novel method for driving a liquid crystal element that solves the problems in conventional liquid crystal display elements or liquid crystal light shutters as described above. It is in.

Another object of the present invention is to provide a method for driving a liquid crystal element having high-speed response.

Another object of the present invention is to provide a method for driving a liquid crystal device having high density pixels.

The present invention provides a. a matrix electrode structure in which pixels are formed at the intersections of a scanning electrode group and a signal electrode group; a liquid crystal cell having a ferroelectric liquid crystal that produces one stable state and a second stable state in response to an electric field of the other polarity, and b. A scan selection signal having one polarity pulse, the other polarity pulse, and one polarity pulse is sequentially applied to the signal electrode group in synchronization with the first phase of the scan selection signal on the selected scan electrode. A voltage signal is applied so that the pixel on the scan electrode becomes an erased state by applying a voltage of one polarity that simultaneously causes the first stable state to the pixel, and the first stable state of the scan selection signal is applied. By applying to the signal electrode group in synchronization with the two phases, a voltage of the other polarity that selectively causes the second stable state in the pixels on the scanning electrode, and a voltage that maintains the erased state. Then, a pulse of one polarity or a pulse of the other polarity is selectively applied to the signal electrode group and the pixels on the scanning electrode at the same time in synchronization with the third phase of the scanning selection signal so that a write state occurs. The scanning selection is performed by applying a voltage signal such that a voltage having a polarity opposite to that of the voltage applied to the pixel in the second phase and maintaining the written state is applied. A liquid crystal device is characterized in that the liquid crystal device has a means for writing to pixels on a scanning electrode that has been selected for scanning, and applying an alternating voltage to pixels on a scanning electrode that are not selected for scanning to maintain the written state at the time of scanning selection. ing.

Note that the polarities of the voltages applied to the scan electrodes and the signal electrodes are based on the voltages applied to scan electrodes that are not selected for scanning.

〔Example〕

The optical modulation substance used in the driving method of the present invention has at least two stable states, and in particular takes either a first optically stable state or a second optically stable state depending on the applied electric field. . That is, a substance having a bistable state with respect to an electric field, particularly a liquid crystal having such a property, is used.

As the liquid crystal having bistability that can be used in the driving method of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, among which chiral smectic C phase (SmC) and
H-phase (SmH〓) liquid crystal is suitable. Regarding this ferroelectric liquid crystal, “Le Giurnard de...”
“Le Journal de
``Physiove letter'') Volume 36 (L-69), 1975 ``Ferroelectric Liquid Crystals''; ``Applied Physics Letters''
Rhysics Letters”) Volume 36 (No. 11) 1980 “Submicron Second Bistable Electro-Optical Switching in Liquid Crystals”
Bistable Electrooptic Switching in Liquid
“Fixed Physics” 16 (141) 1981 “Liquid Crystals”
The ferroelectric liquid crystal disclosed in these documents can be used in the present invention.

More specifically, as an example of the ferroelectric liquid crystal compound used in the method of the present invention, decyloxybenzylidene-p'-amino-2-methylbutyl cinnamic
(DOBAMBC), hexyloxybenzylidene-p'-amino-2-chloropropylcinnamate (HOBACPC) and 4-o-(2-methyl)
-butylresolcylidene-4'-octylaniline (MBRA8) and the like.

When constructing an element using these materials, the element is supported by a copper block or the like with a heater embedded in it, as necessary, in order to maintain the temperature at which the liquid crystal compound becomes the SmC〓 phase or SmH〓 phase. can do.

Further, in the present invention, it is also possible to use ferroelectric liquid crystals that appear in chiral smect F phase, I phase, J phase, G phase, or K phase other than the above-mentioned SmC〓 and SmH〓.

FIG. 2 schematically depicts an example of a ferroelectric liquid crystal cell. 21a and 21b are In ₂ O ₃ , SnO ₂ or
It is a substrate (glass plate) coated with a transparent electrode such as ITO (indium-tein-oxide).
In between, a SmC phase liquid crystal oriented such that a liquid crystal molecular layer 22 is perpendicular to the glass surface is sealed. A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment (P⊥) 14 in a direction perpendicular to the molecule. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 21a and 21b, the helical structure of the liquid crystal molecules 23 is unraveled, and the alignment direction of the liquid crystal molecules 23 is changed so that all dipole moments (P⊥) 24 are directed in the direction of the electric field. can be changed. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, polarizers arranged in a crossed nicol position can be placed above and below the glass surface. For example, it is easily understood that the liquid crystal optical modulation element is a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ), the helical structure of the liquid crystal molecules unwinds even when no electric field is applied, as shown in Figure 3, and its dipole moment (Pa or
Pb takes either an upward direction 34a or a downward direction 34b. When an electric field Ea or Eb of different polarity above a certain threshold value is applied to such a cell for a predetermined period of time as shown in Figure 3, the dipole moment becomes
The liquid crystal molecules are oriented either upward 34a or downward 34b in accordance with the electric field vector 24Ea or Eb, and accordingly, the liquid crystal molecules are aligned in either the first stable state 33a or the second stable state 33b.

There are two advantages to using such a ferroelectric liquid crystal as an optical modulation element. Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has a bistable state. To explain the second point with reference to FIG. 2, for example, when the electric field Ea is applied, the liquid crystal molecules are oriented in the first stable state 33a, but
This state remains stable even when the electric field is turned off. Furthermore, when an electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 33b and the orientation of the molecules is changed, but they remain in this state even after the electric field is turned off. Further, as long as the applied electric field Ea does not exceed a certain threshold value, each orientation state is maintained. In order to effectively realize such fast response speed and bistability, it is preferable for the cell to be as thin as possible, generally 0.5μ to 20μ, especially
1μ to 5μ is suitable.

Preferred specific examples of the driving method of the present invention will be explained with reference to FIGS. 4 to 7.

FIG. 4 is a schematic diagram of a cell 41 having a matrix electrode structure in which a ferroelectric liquid crystal compound is sandwiched between. 42 is a scanning electrode group, and 43 is a signal electrode group. Now, to simplify the explanation, an example will be shown in which a black and white binary signal is displayed. In FIG. 4, it is assumed that the pixels indicated by diagonal lines correspond to "black" and the others correspond to "white". FIGS. 5a and 5b respectively show a scan selection signal applied to a selected scan electrode and a scan non-selection signal applied to other scan electrodes (unselected scan electrodes),
FIGS. 5c and 5d represent information selection signals applied to selected signal electrodes and information non-selection signals applied to unselected signal electrodes, respectively. Figure 5 a~
In b, the horizontal axis represents time and the vertical axis represents voltage.

Now, the threshold voltage at the application time Δt to give the first stable state (this is defined as white) of a crystal cell having bistable property is −Vth ₁ , and the second stable state (this is defined as black) is set as −Vth 1. ), the electric signal applied to the selected scanning electrode is 2V ₀ at phase (time) t 1, and 2V ₀ at phase (time) t ₁ , and (time) t ₂
The voltage waveform is -2V ₀ at phase t ₃ and V ₀ at phase t ₄ and 0 at phase t 4 .

Further, the other scanning electrodes are in a grounded state as shown in FIG. 5b, and the electric signal is 0. On the other hand, the electric signal applied to the selected signal electrode is −V ₀ at phase t ₁ as shown in FIG.
At phase t ₂ it is V ₀ , at phase t ₃ it is 0 and at phase t ₄ it is V ₀ . Also, the electrical signal applied to the unselected signal electrodes is -V 0 at phase t ₁ and -V ₀ at phase t ₂ as shown in FIG. 5d.
V ₀ , 0 at phase t ₃ and V ₀ at t ₄ . In the above, the voltage value V ₀ is V ₀ < Vth ₂ < 3V ₀ and −V ₀
>-Vth ₁ >-3V ₀ is set to a desired value. FIG. 6 shows the voltage waveform applied to each pixel when such an electric signal is applied. 6th
In the figure, a and b are on the selected scanning electrode line, respectively, and the pixels to display "black" and "white", and c and d are on the unselected scanning electrode, respectively. This is the voltage waveform applied to the pixel. As is clear from FIG. 6, all the pixels in the selected scan electrode first have a single charge because a voltage _{−3V 0 exceeding the threshold voltage −Vth 1 is} applied in the first phase t 1 . Aligned to white. Therefore, this phase t ₁ corresponds to the line erasure phase. Among these, the pixel that should be displayed as "black" has the second phase _t2 ,
A transition to the other optically stable state (“black”) occurs because a voltage 3V ₀ exceeding the threshold voltage Vth ₂ is applied.
Furthermore, in the case of a pixel that exists on the same scanning electrode and is to be displayed as "white", the applied voltage in the second phase t ₂ is a voltage V ₀ that does not exceed the threshold voltage Vth ₂ . It remains in a stable state.

On the other hand, on unselected scanning electrodes, the voltage applied to all pixels is ±V ₀ or 0,
Neither exceeds the threshold voltage. Therefore, the liquid crystal molecules maintain the orientation corresponding to the signal state when scanned last time without changing the orientation state. That is, when a scanning electrode is selected, first in the first phase t ₁ it is aligned to one optically stable state "white", and then in the second phase t ₂ the selected pixel is aligned It is transferred to the other optically stable state (black), one line of signals is written, and that signal state can be maintained until the next frame is selected. .

〔Effect of the invention〕

According to the present invention, even if a display panel using a ferroelectric liquid crystal element is driven at high speed, the write state in the second phase t ₂ after the erase state in the first phase t ₁ in the third phase t ₃ In particular, it was possible to improve the stabilization of the state in which the erased state is maintained.

Preferably, the maximum pulse width of the voltage waveform that continues to be applied to the pixels of the scan electrode line to which the scan non-selection signal is applied is twice the pulse Δt during writing, so that the display is performed during the writing scan of one screen. It is possible to effectively prevent the phenomenon that the state is reversed to another display state.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the threshold characteristics of a ferroelectric liquid crystal. FIGS. 2 and 3 are perspective views schematically showing a ferroelectric liquid crystal element used in the present invention. FIG. 4 is a plan view of the matrix pixel structure used in the present invention. FIGS. 5a to 5d are explanatory diagrams showing voltage waveforms of signals applied to the electrodes, respectively. FIGS. 6a to 6d are explanatory diagrams showing voltage waveforms of signals applied to pixels, respectively. FIG. 7 is an explanatory diagram of voltage waveforms representing the aforementioned signals in time series.

Claims (1)

[Scope of Claims] 1a A matrix electrode structure in which pixels are formed at intersections of a scanning electrode group and a signal electrode group, and a matrix electrode structure arranged between the scanning electrode group and the signal electrode group and resisting an electric field of one polarity. a liquid crystal cell having a ferroelectric liquid crystal that produces a first stable state in response to an electric field of the other polarity and a second stable state in response to an electric field of the other polarity; A scan selection signal having one polarity pulse, the other polarity pulse, and one polarity pulse is sequentially applied in this order, and in synchronization with the first phase of the scan selection signal, the selected scan electrode is applied to the signal electrode group. Applying a voltage signal such that the pixels on the scanning electrode enter the erased state by simultaneously applying a voltage of one polarity that causes the first stable state to the pixels above, and applying the voltage signal to the scanning selection signal. A voltage of the other polarity that selectively causes the second stable state in the pixels on the scanning electrode and a voltage that maintains the erased state are applied to the signal electrode group in synchronization with the second phase of the scanning electrode. By selectively applying a pulse of one polarity or a pulse of the other polarity so that a write state occurs, and synchronizing with the third phase of the scanning selection signal, the pixel on the scanning electrode is applied to the signal electrode group. At the same time, by applying a voltage signal such that a voltage having a polarity opposite to that of the voltage applied to the pixel in the second phase and maintaining the written state is applied. Writes to the pixels on the scan electrodes selected for scanning, and writes to the pixels on the scan electrodes not selected for scanning.
A liquid crystal device comprising: means for applying an alternating voltage to maintain a written state during scan selection;